Many public, commercial and office buildings have illuminated signs delineating a path to an exit. A common example of such an illuminated sign is the "EXIT" sign 2 illustrated in FIGS. 1 and 2, which has an opaque front face 4 and light transmissive (translucent) indicia 6 such as letters and/or symbols thereon. Standard electrical lamp sockets 8 are positioned inside the sign 2 for securing and lighting lamps 10 therein to illuminate the translucent indicia 6. Although the present invention applies to any illuminated sign, this discussion will be limited to the illustrated "EXIT" sign of FIGS. 1 and 2 for simplicity.
Most "EXIT" signs in use today are illuminated by an incandescent or florescent lamp 10 threaded into two internally mounted sockets 8. The translucent letters typically include a filter so that light emanating from the sign has the desired color, such as red or green letters and arrows to indicate exit path locations. These signs can be one sided as shown in FIGS. 1 and 2, or two sided with opposing front and rear faces having translucent indicia thereon.
Conventional illuminated signs have several drawbacks. The standard incandescent or fluorescent lamps used therein have a relatively short life span and consume significant amounts of energy. These problems are of special concern because most "EXIT" signs are continuously illuminated. Another drawback with incandescent or fluorescent lamps is that the translucent portions of the illuminated sign are not uniformly illuminated because the portions of the translucent indicia closer to the lamps are generally brighter than those portions further away.
To reduce electrical cost and increase reliability, Light Emitting Diode (LED) lamps have been developed to replace the conventional incandescent or fluorescent lamps typically used in existing "EXIT" signs. LED lamps consume less energy than convention lamps while exhibiting much longer lifetimes. However, the LED lamp designs used thus far still do not provide sufficiently bright, uniform illumination of the translucent portions of the sign face(s).
For Example, U.S. Pat. No. 5,388,357 discloses LED arrays mounted internally on the interior sign box side using an adhesive, and adapters electrically connecting the lamp sockets to the LED arrays. U.S. Pat. No. 5,459,955 discloses linear array LED lamps having a standard incandescent lamp base for direct replacement into the lamp socket. The resulting illumination over the face of the sign for either of these designs, however, is insufficiently uniform because brighter illumination occurs for those portions of the sign face directly opposing the output of the LEDs.
Other schemes have been developed to produce a more even illumination with LED based lamps, including indirect illumination using a reflector as shown in U.S. Pat. No. 5,428,912, and a plurality of fibers depicting the word "EXIT" in U.S. Pat. No. 5,040,320. These designs, however, are complex, impractical and/or too costly for retrofitting existing "EXIT" signs. Further, these designs still fail to provide sufficiently uniform illumination over the sign face.
There is a need for a simple LED retrofit lamp that conveniently installs into existing "EXIT" signs, and which brightly and uniformly illuminates the translucent indicia on one or more faces of the illuminated sign.